The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, a hybrid electric vehicle is a vehicle which uses two different power sources efficiently.
Such a hybrid electric vehicle typically uses an engine and a motor/generator. The hybrid electric vehicle uses the motor/generator having relatively better low-speed torque characteristics as a main power source at a low-speed and uses an engine having relatively better high-speed torque characteristics as a main power source at a high-speed.
Since the hybrid electric vehicle stops operation of the engine using the fossil fuel and instead uses the motor/generator at a low-speed region, fuel consumption efficiency may be improved and exhaust gas may be reduced.
The power transmission system of a hybrid electric vehicle is classified into a single-mode type and a multi-mode type.
A torque delivery apparatus such as clutches and brakes for shift control is not necessary, but fuel consumption is high due to deterioration of efficiency at a high-speed region and an additional torque multiplication device is desired for being applied to a large vehicle according to the single-mode type.
Since the multi-mode type has high efficiency at the high-speed region and is able to multiply torque autonomously, the multi-mode type can be applied to a full size vehicle.
Therefore, the multi-mode type instead of the single-mode type is applied as the power transmission system to a hybrid electric vehicle and is also under continuous investigation.
The power transmission system of the multi-mode type includes a plurality of planetary gear sets, a plurality of motor/generators operated as a motor and/or a generator, a plurality of torque delivery apparatus controlling rotation elements of the planetary gear sets, and a battery used as a power source of the motor/generators.
The driving mode of the power transmission system having the multi-mode type can be divided into an EV mode (electric vehicle mode) that only uses the motor/generator as a power source and a HEV mode (hybrid electric vehicle mode) that uses the engine and the motor/generator as a power source.
Further, the HEV mode can be divided into a power split mode and an over drive mode (engine coupling mode) according to a type of the power transmission system. We have discovered that if the driving mode of the power transmission system is appropriately converted from the power split mode to the engine coupling mode, it is possible to improve driving efficiency.
Therefore, converting method of the driving mode has been studied.